President Obama says it all the time — no combat troops will return to Iraq.

But many experts believe it will be extremely hard to achieve Obama’s newly expanded military mission there without more Americans on the ground.

“I think the slippery slope analogy is the right one for Iraq right now,” said Barry Posen, director of the Security Studies program at the Massachusetts Institute of Technology.

On Thursday, Obama authorized a new open-ended operation in response to gains by the Islamic State militants in northern Iraq.

For now, the new mission relies on aircraft based outside Iraq. The U.S. will help defend the Kurdish city of Erbil from Islamic State fighters using “targeted air strikes,” Obama said. Those air strikes began Friday morning and included at least three separate bombings before noon, defense officials said.

The second mission is a commitment to protect some 40,000 Iraqi Yazidis who are trapped on a mountain surrounded by the militants. That began Thursday night with air drops of food and water for at least 8,000 people.

Military experts say tactical commanders will want more ground forces. Forward air controllers could provide more precise targeting information. U.S. advisers could support the Kurdish forces fighting the militants. And U.S. commanders may need to expand their intelligence effort on the ground.

In turn, U.S. forces might need a forward operating base with a security perimeter, more force protection and a logistical supply line. Medevac capabilities may require a helicopter detachment and a small aviation maintenance shed.

“You’re talking about a 10,000- to 15,000-soldier effort to include maintenance, and medevac and security,” said retired Army Col. Peter Mansoor, who served as executive officer to David Petraeus during the 2007 surge in Iraq and now is a professor of military history at Ohio State University.

“But that is the price you’re going to pay if you want to roll back [Islamic State]. You can’t just snap your fingers and make it go away,” Mansoor said.

Obama’s address to the nation Thursday night suggested that the city of Erbil will be a no-go zone for the militants, and he offered no timeframe for that commitment.

The biggest near-term military challenge stems from Obama’s commitment to prevent a “genocide” of the Yazidi people trapped on Mount Sinjar. The air drops providing food and water that began Thursday night are a short-term solution. Obama promised to use air strikes on Islamic State forces, if needed, to “break the siege” and “help refugees get the shelter and food and water they so desperately need.”

Getting the Yazidis off the mountain and safely transporting them to a secure location will require either an “an enormous helicopter air lift” or ground combat units to confront militants and secure a safe-passage corridor for the refugees, Mansoor said.

“That may require some kind of ground presence to escort them through enemy held territory,” Mansoor said.

“That is [IS] controlled territory. There could be major combat along the way. This could be very difficult,” Mansoor said.

'Leverage locals'

The key to limiting ground-level involvement for U.S. service members will be coordinating with the Kurdish Peshmerga militia or other allied forces, said Mark Gunzinger, a retired Air Force colonel and senior fellow at the Center for Strategic and Budgetary Assessments.

“I don’t think this is headed down a slippery slope whatsoever,” Gunzinger said in an interview Friday.

He pointed to the success at the early stages of the Afghanistan war in 2001 when U.S. aircraft, working closely with small U.S. special operations teams and friendly Afghan forces, toppled the Taliban regime.

“I think that kind of model could be effective in Iraq,” Gunzinger said.

That model may be even more effective today because the U.S. military has far more drones to provide a constant presence overhead.

Also, there is little evidence that Islamic State forces have significant anti-aircraft weaponry, making an aggressive U.S.-led air campaign easier, Gunzinger said.

The analogy of Afghanistan in 2001 was also cited by Seth Jones, a counter-insurgency expert with the Rand Corporation. “Leveraging locals is the key,” Jones said in an interview Friday, adding that additional ground forces may be limited to small elements of forward air controllers and special forces teams.

While the need for U.S. ground troops may be limited, Jones said, Obama’s plan poses another risk: If air strikes are successful in the area around Erbil, pressure may grow for the U.S. to provide similar air strikes in other parts of Iraq. “The slippery slope may be a much broader demand for air strikes,” Jones said.

It’s unclear how far Obama and his military leaders plan to take this current campaign.

“There is still some question about whether this is going to be a major air campaign to defeat [the Islamic State] or whether it is going to me more along the lines of strikes and raids to deny them access and prevent them from making further advances. I’m not sure,” Gunzinger said.

Obama’s language Thursday was ambiguous, Posen said. Despite his repeated aversion to sending “combat troops” back into Iraq, Obama has signaled a long-term commitment to support the Iraqi military and a continued belief in a cohesive, Democratic Iraq in which Sunnis and Shiites and Kurds share power under a Bagdad led government.

“Is this going to be a limited mission? Or is this the beginning of a project where we are once again going to fix Iraq, to build a homogenous, unified Iraq?” Posen said. .

“If they are going to succumb to that logic, if they are going to try to build the beautiful outcome that the Bush Administration failed to build, then they are not edging up to the slippery slope — they are diving over it.”